1. Field of the Invention
The present invention relates to spatial phase modulating masks which are particularly useful as an exposure mask in the production of phase-shifted diffraction gratings of distributed feedback (DFB) semiconductor lasers. The present invention also relates to a process for the production of such spatial phase modulating masks and a process for the formation of phase-shifted diffraction gratings using such masks. The resulting single longitudinal mode for the DFB lasers is very important in the field of optical communication, since it can prevent a distortion of the waveform due to wavelength dispersion, can reduce noise, and can increase the utility thereof in applied optical instrumentation.
2. Description of the Related Art
Considerable development work is underway in general on semiconductor lasers with oscillation wavelengths of 1.5 to 1.6 micrometers due to the minimal loss of light of that wavelength band in transmission over optical fibers.
If a semiconductor laser of this conventional type, i.e., a Fabry-Perot (FP) semiconductor laser, is used for a high speed modulation, it cannot maintain the wavelength monochromatically and numerous wavelengths result.
If such a signal light is introduced into and transmitted through an optical fiber, the light output therefrom results in a degradation of the waveform because the refractive indexes, and thus the propagation speeds, for respective wavelengths are different due to the differences in dispersion in the material of the optical fiber itself.
Such a signal, therefore, is received at the receiving side with a great amount of noise, and so is not practical for use.
In recent years, therefore, development has been underway on DFB semiconductor lasers and good results have been obtained.
A DFB type semiconductor laser, formed on the active layer itself or layer adjacent thereto, has a diffraction grating known as a "corrugation" or just a "grating", and light travels back and forth and resonates in the active layer under the influence of this diffraction grating.
In such a DFB semiconductor laser, theoretically, it is considered possible to maintain a monochromatic wavelength oscillation even when modulating at a high speed of several hundred Mbits/sec. In practice, however, this is very difficult.
This is because the corrugations in the afore-mentioned DFB laser are formed uniformly and, therefore, the corrugations have a uniform structure without discontinuity. In other words, a so-called symmetric DFB semiconductor laser is formed in which, since the losses in the two longitudinal modes symmetrically occurring on the two sides of the side center are equal, dual-mode oscillation can take place or oscillation can transfer between two resonance modes differing by just plus or minus the same wavelength from the Bragg wavelength corresponding to the period of the corrugations, resulting in unstable oscillation.
Therefore, a so-called .LAMBDA./2-shifted DFB semiconductor laser (.LAMBDA.=corrugation period) has been developed to eliminate this problem. A conventional .LAMBDA./2-shifted DFB type semiconductor laser has a structure in which, seen from the side center, the corrugation of the right side section or the left side section is shifted by just .LAMBDA./2. The .LAMBDA./2-shifted DFB semiconductor laser can oscillate with a single mode at the Bragg wavelength. The oscillation characteristics of the .LAMBDA./2-shifted DFB semiconductor laser are extremely superior.
There are, however, considerable problems in the manufacture of the .LAMBDA./2-shifted DFB semiconductor laser. Specifically, the period .LAMBDA. of the corrugation itself is as small as 0.3 to 0.4 micrometers, for example. Therefore, it is very difficult to manufacture the right and left two corrugations as being shifted by exactly .LAMBDA./2 and being combined at the middle of the DFB laser without discontinuity of the corrugations.
Recently, several improved methods have been proposed to realize satisfactory quarter-wave (.lambda./4)-shifted or similar phase-shifted DFB semiconductor lasers. One such method is to fabricate phase-shifted corrugations using electron-beam lightography. K. Sekaptedjo, et al reported in their Electronics Letters, Jan. 19, 1984, Vol. 20, No. 2, pp. 80-81 that:
"We fabricated phase-shifted DFB lasers. The second-order corrugation was formed on the InP substrate using an electron-beam exposure system with a precise pitch controller, where at the centre the phase of the corrugation was shifted by .LAMBDA./4, corresponding to a shift of .pi. in the first-order space-harmonics. The corrugation was transcribed into InP substrate by etching with HBr+HN0.sub.3 +10 H.sub.2 O. The liquid-phase epitaxy was carried out to grow the n-GaInAsP (.lambda..sub.g =1.35 .mu.m) buffer, undoped GaInAsP (.lambda..sub.g =1.55 .mu.m) active, p-InP cladding and p-GaInAsP cap layers successively, where a GaAs cover was used to preserve the corrugated surface from thermal deformation. Lasers with 30 .mu.m oxide stripes were cut with sawed sides, and the cavity was formed by sawing at one end to suppress the Fabry-Perot modes and cleaving the other end to provide the output facet, so that the point of the phase shift is at the centre of the cavity." The use of electron-beam lithography in the phase-shifted corrugations suffers from some drawbacks. For example, it requires complicated and troublesome operations, and takes a large amount of production time. In addition, it is not suitable for a mass production of the corrugations.
Another improved method is to fabricate .lambda./4-shifted InGaAsP/InP DFB lasers by simultaneous holographic exposure of positive and negative photoresists. K. Utaka, et al reported in their Electronics Letters, Nov. 22, 1984, Vol. 20, No. 24, pp. 1008-1009 that:
"First, a negative photoresist (OMR) with a thickness of about 700 .ANG. and a positive photoresist (MP) were spin-coated successively on an n-InP substrate. Parts of the upper positive photoresist were removed by the conventional photolithography and the negative photoresist in the disclosed area was etched off by using sulphuric acid. Second, after removing the remaining positive photoresists, .about.700 .ANG.-thick positive photoresist was newly coated on a whole surface. Consequently, some parts of the substrate were covered with positive photoresist and the other parts with negative and positive photoresists. These separately deposited photoresists were simultaneously exposed by the holographic exposure using 3250 .ANG. He-Cd laser. After the developments and the transcriptions carried out separately for each photoresist, .lambda./4-shifted corrugations were formed."
It should be noted that the combined use of positive and negative photoresist is a good idea, but, in the practice of this method, it is difficult to find specific photoresists, particularly negative photoresists, having a high resolving power. Further, steps in a separate coating and simultaneous patterning of the positive and negative photoresists are complicated and troublesome.